1. Field of the Invention
The present invention relates to a temperature control system and an ink jet printer utilizing the temperature control system, and more particularly to a temperature control system in an ink jet printer having a plurality of ink jet units and an ink jet printer utilizing such a temperature control system.
2. Related Background Art
In a color ink jet printer having a plurality of head units having a plurality of nozzles, it is rare that all head units are uniformly driven, and in many cases, a head unit which discharges ink of a particular color is more frequently driven than other head units.
Many head drive systems are known. When head units which discharge inks by thermal energy are used, the head units are accompanied by a temperature increase whatever drive system may be used.
The more frequently the head unit is driven, the larger is the temperature increase. Thus, the temperature charges from head unit to head unit. The temperature increase of the head unit is also caused by heat generated by printer transformers, transistors and motors.
The ink jet printer is usually provided with a cooling fan to prevent the temperature increase.
The ink jet printer head has the following two temperature requirements. The first requirement is an operable temperature range (environment temperature range in which the printer is operable) defined by a specification. It may be 0-40.degree. C.
The other requirement is a stable discharge temperature range of the head. It may be 15-50.degree. C. The lower limit of 15.degree. C. of the stable discharge temperature range means that if the temperature is lower than the lower limit, the viscosity of the ink is lowered and stable discharge is not attained. The upper limit of 50.degree. C. means that if the temperature is higher than the higher limit, gas in the ink separates so that the ink cannot be stably discharged and durability of the head parts is lowered. Those limits are experimentarily determined and vary with material and property of the ink, and reliability, characteristic and required performance of the apparatus.
However, because of the temperature requirements described above, it is necessary to heat the printer by a heater when the temperature is low and cool the printer by a fan when the temperature is high.
In prior art printers, a temperature sensor is used to sense a temperture of the entire head unit, and if it is lower than a predetermined temperature, the heater is energized, and if it is higher than another predetermined temperature, the fan is energized.
However, not all head units are uniformly driven to raise temperature as described above but the temperature rise varies from head unit to head unit. The fans are not provided one for each of the head units because of space and cost considerations. As a result, the head unit which does not need heating or cooling may be heated or cooled and the temperature range described above may not be maintained.
When a heater having a high heating power is used to keep the head units at the desired temperature even if the fan is energized, power consumption increases or ink in the head unit which is not properly cooled is abruptly heated too much and durability and recording characteristic are deteriorated.
When a heat transfer coefficient of the fan is high, the cooling effect will be sufficient but the size of the fan increases, which leads to increases of size and weight of the printer and increase of power consumption. In addition, a fan having a cooling power which exceeds a heater power may be selected.
When the fan and the heater are variably controlled. the control must be very precise and hence a complex control circuit is required.
When a high heating power heater and a high cooling power fan are used, they are frequently turned on and off in order to keep the temperature in the desired range, and it is difficult to attain a precise control, or even if it is possible, the cost may be very high.